Optical telecommunications systems may use single-mode or multimode optical fibers to guide light having data modulations imposed thereon so as to transmit information over a distance. Typically the transmission distance is limited by such factors as power loss in the fiber, and the dispersion or distortion of the signal due to intrinsic properties of the optical waveguide, or imperfections in the fiber due to manufacturing defects, the environment, or non-linear effects such as four-wave mixing. Two types of linear distortion mechanisms are chromatic and polarization mode dispersion.
Chromatic, or intramodal, dispersion occurs in both single mode and multimode optical fibers. Chromatic dispersion occurs because different wavelengths of light travel through the fiber waveguide at different speeds. Since the different wavelengths of light have different velocities, some wavelengths of a signal arrive at the fiber end before others. This delay difference leads to pulse broadening.
In an ideal optical fiber, the light-guiding core has a perfectly circular cross-section, and the fundamental electromagnetic propagating mode may be described as having two orthogonal polarizations that travel at the same velocity at each wavelength. The orientation of the polarization axes with respect to the local fiber axis is not a consideration, as the two polarizations propagate with identical properties due to the circular symmetry of the fiber. However, in practice, this ideal state is not achieved, and there are asymmetrical propagation properties with respect to the polarization components.
Symmetry-breaking imperfections fall into several categories: geometric asymmetry: e.g., slightly elliptical cores; and, stress-induced material birefringence. These can arise from either imperfection in manufacturing (which is never perfect or stress-free) or from thermal and mechanical stresses imposed on the fiber when installed is a system. The latter stresses may vary with time, for example, as the temperature changes. These effects cause the polarization and delay of a signal transmitted over a distance to vary as a function of wavelength and time in an apparently random manner, and is called polarization mode dispersion (PMD).
PMD is a factor limiting the upgrade of existing optical fiber communication systems and on the transmission bandwidth in new designs. Traditional PMD compensators typically work in a low-order (first- and second-order as function of wavelength) PMD approximation. In the first-order approximation, the effect of PMD is modeled as a birefringence with frequency-independent magnitude and with frequency-independent axes (which are often called the principal states of polarization (PSP)). However, as the bandwidth of telecommunication systems increases, higher-order PMD effects become increasingly important, and the PSPs and the magnitude of the equivalent birefringence become strongly frequency dependent.